Superchargers and turbochargers are well known devices for increasing the performance of internal combustion engines. These devices include an impeller to drive more air (and thus oxygen) into the combustion chamber to “boost” performance. The impeller is typically housed in a volute chamber which directs the air flow into the combustion engine.
The ever increasing demands from the motor-sport consumer require higher boost pressures and flow rates from highly efficient superchargers. This level of performance is achieved by very high rotational speeds, some in excess of 60,000 RPM. To get the desired performance from a supercharger, the impeller is subjected to extreme stresses as set out in more detail below.
Those whom are expert in the field will recognize that the centrifugal forces exerted on the impeller are more extreme as rotational speeds increase. Those whom are expert in the field will also recognize that tangential stress, otherwise known as ‘hoop stress’, experienced by the outer portions of the hub and shroud, far exceeds the radial stress that is experienced by the vanes. It is for this reason, in addition to preferable pressure/flow characteristics, that impellors designed for high speed rotational operation must have vanes that are perpendicular to the rotational axis, as they are critical in reinforcing the impeller, and thus having a rake angle that is greater or less than zero degrees will diminish the strength of the vane on the impeller. What is desired is an impeller design which can perform efficiently while withstanding the high stresses due to the high rotational speeds required for its intended use.
Previous art (Genster, Albert—U.S. Pat. No. 6,033,183)(Hirose et al—U.S. Pat. No. 6,592,329 B1) (Chapman, Albert—US-2005/0163614 A1) claims vanes that are reinforced by the hub and shroud, thus contradicting the tangential/radial stress relation mentioned above. This makes them unstable and thus unsuitable for high speed operation as would be required for applications such as a supercharger or a high pressure blower. Previous art (Harada et al.—U.S. Pat. No. 6,338,610 B1)(Nikpour, Bahram—US—2005/0196272 A1) (Chapman, Thomas R.—US-2005/0163614 A1), (The Garrett Corp.—GB-761937), (W U, Chiang-Fu—DE4030817 A1), (Nikolaev Yu et al.—RU-2183772) show vanes that have a zero rake angle at the central portion of the impeller which then proceeds to a negative rake angle at the outer periphery, or have a negative rake angle the whole length of the vane. These designs are also likely to fail when applied to superchargers for the reasons mentioned above as the vane rake angle at their periphery is greater/less than zero and thus weakened. In addition, as is commonly known by those who are expert in the field, an impeller with vanes that are negatively raked will exhibit pressure/flow characteristics in which pressure drops as flow increases, and for this reason they are unsuitable for use in a supercharger as many of them claim. This is because the moment the throttle is opened and the air flow increases, the boost pressure will drop. The negatively sloping curve on the pressure/flow charts in previous art (Harada at al.—U.S. Pat. No. 6,338,610 B1 FIG. 8) shows this. In a supercharger application it is preferable that the level of boost increases as flow increases, or at the very least remain the same.